Rocket sits at the Numbers Desk with two fingers on his wrist. "Shh. I am counting."
"Counting what?" Raven asks. "Heartbeats," Rocket says. "Eighteen in 15 seconds. That is a rate."
Raven writes it down. "Heartbeats per 15 seconds. Can you make it per minute?"
"Four fifteens in a minute," Rocket says. "So 18 times 4. Seventy-two beats per minute."
Nova circles the kitchen slowly. "Rates are everywhere here. What do you notice about the dripping tap?"
Raven listens. "One drop every 2 seconds. That is a rate too, and a very annoying one."
A rate is anywhere two different units go together: heartbeats per minute, pages per minute, drops per second.
A unit rate makes any two rates easy to compare. Seventy-two beats per minute can be compared with any other count.
Changing units is ratio reasoning too. Four groups of 15 seconds make 1 minute, so multiply a 15-second count by 4.
| Number sentence | True or false? |
|---|---|
| 18 × 4 = 72 | ? |
| 56 ÷ 8 = 7 | ? |
| 36 ÷ 9 = 5 | ? |
| 250 ÷ 100 = 2.5 | ? |
| 2 × 60 = 100 | ? |
Next week the crew counts Field Day sign-ups. A percent is a special rate: a number per 100.
If 10 of 40 kids choose the relay, that is 25 per 100, or 25 percent. You already know how to scale it.
A full week of rates. Tomorrow is Data Day: the family counts heartbeats and steps.